The field of the invention relates to expanded polystyrene insulation material, and more particularly to insulation material produced from admixtures of expandable polystyrene beads with binders or fillers.
Polystyrene is produced in an expandable granular form referred to as polystyrene "beads". The beads have a closed cellular structure and contain an expanding agent such as pentane. The beads may be formed into integrated bodies, such as sheets or slabs, by a molding process in which steam and heat are applied to cause the beads to soften, expand, and integrate. In most applications, the beads are subjected to a pre-expansion before being formed into the molded bodies, the beads being only partially expanded during the pre-treatment. Polystyrene insulating board as presently produced from such beads usually does not contain any additional binders or fillers. However, the patent literature contains some descriptions of such composites. See, for example, U.S. Pat. Nos. 3,151,604; 3,251,916; and 4,079,170.
In the process described in U.S. Pat. No. 3,151,604, pre-expanded polystyrene beads are combined with a thermol plastic polymeric bonding agent that is adapted to flow at a temperature lower than the expansion temperature range of the beads. This mixture is mechanically preformed at a temperature at which the thermoplastic is flowable but below a temperature at which the beads expand. Thereafter, the preformed mass is subjected to heat to expand the beads.
The U.S. Pat. No. 3,251,916 discloses the formation of composite bodies from expanded polystyrene and a polyurethane binder. Other disclosed bonding agents include drying oils, varnishes, and rubber latex gels, which are to be used together with the polyurethane. The polystyrene beads are expanded before being mixed with the binder. U.S. Pat. No. 4,079,170 describes the impregnation of a pre-formed sheet of expanded polystyrene with a wax composition, liquified wax being forced into the sheet under pressure and hardened by cooling.
One of the known disadvantages of expanded polystyrene insulation is that it is a flammable material which can contribute to fire spreading. The incorporation of fire retardant chemical has been proposed, but such chemicals increase the cost of the insulating material. Further, the incorporation of fire retardant chemicals may interfere with the self-bonding of the polystyrene beads during the manufacture of the board, thereby reducing the mechanical strength of the insulation and increasing its friability. Heretofore, no method has been proposed whereby the fire resistance of polystyrene bead insulation can be appreciably increased at minimal additional cost, and with substantial improvement in other desirable properties.